(a) Field of the Invention
The present invention relates to electrophotographic photoreceptors, more particularly, to electrophotographic photoreceptors which maintain high mechanical strength and excellent electrophotographic properties for a long period. The electrophotographic photoreceptors are useful in various fields of electrophotography.
(b) Description of the Related Art
In the fields of electrophotography, recently, there have been mainly used organic electrophotographic photoreceptors including layered-type electrophotographic photoreceptors and single-layer-type electrophotographic photoreceptors. The layered-type electrophotographic photoreceptors have a photosensitive layer comprising at least two elementary layers, that is, a charge generation layer where charges are generated by exposure, and a charge transport layer where transporting of the potential occurs. In the layered-type electrophotographic photoreceptors, the charge transport layer is composed of binder-resins and charge transporting materials dispersed or dissolved in the binder-resins. The single-layer-type electrophotographic photoreceptors have a photosensitive layer comprising one elementary layer where charge generating materials and charge transporting materials are dispersed or dissolved in binder-resins. Polycarbonate resins prepared from bisphenol A as a raw material have been widely used as the binder-resins both in the charge transport layers of layered-type electrophotographic photoreceptors and in the photosensitive layers of the single-layer-type electrophotographic photoreceptors.
Polycarbonate resins made from bisphenol A have such advantageous characteristics that they have relatively high mechanical strength and that they provide product photoreceptors with good electrical properties because of their good compatibility with charge transporting materials.
The use of polycarbonate resins made from bisphenol A for forming the charge transport layer of the photosensitive layer, however, encountered the following problems (1) and (2).
(1) In preparation of photoreceptors, whitening (gelation) of coating solutions for forming charge transport layers or photosensitive layers tends to occur or the formed charge transport layers or photosensitive layers tend to crystallize easily, depending on the solvents used for preparation of the coating solutions. This crystallization causes quality defects of developed images since photo-induced discharge hardly occurs on the crystallized regions of the charge transport layer, leaving residual charges which cause an undesirable electric potential on the regions.
(2) Charge transport layers or photosensitive layers formed by using polycarbonate resins made from bisphenol A have disadvantages that they tend to be peeled off from base layers owing to their poor adhesion to the base layers and that they are apt to be damaged or worn out owing to their poor surface hardness, resulting in their short lives in practical use for copying. Herein, the terms "base layer" mean the charge generation layer in general layered-type electrophotographic photoreceptors or the electroconductive substrate in single-layer-type electrophotographic photoreceptors. In case of positively-charged-type electrophotographic photoreceptors where a charge transport layer and a charge generation layer are successively laminated on an electroconductive substrate in that order, the terms "base layer", however, mean the electroconductive substrate. In case a blocking layer or an intermediate layer is interposed between an electroconductive substrate and a charge transport layer or a photosensitive layer or between a charge generating layer and a charge transport layer in order to improve electrophotographic properties, the terms "base layer" mean the blocking layer or the intermediate layer.